1. Field of the Invention
The present invention relates to a regenerative material which exhibits a large specific heat at a low temperature.
2. Description of the Related Art
In recent years, the technology of devices used in association with superconductor materials has advanced remarkably and has been applied to more and more technical fields. Along with the increasing use of superconductor technology, demands are increasing for a high-efficiency, small refrigerator for cooling superconductive components. There is a significant demand for a refrigerator which is light and small and has a high heat-efficiency. At present, such refrigerators are being developed in two ways. The first method is to enhance the efficiency of the existing gas-cycle refrigeration devices by adopting, for example, the Stirling cycle. The second method is to employ a new refrigerator in place of conventional gas-cycle refrigeration. A new refrigerator includes those using a heat-cycle, such as a Carnot-type and an Ericsson-type cycle, and the magnetocaloric effect.
Among the gas-cycle refrigerators with enhanced efficiency are: refrigerators which operate using the Stirling cycle and refrigerators which operate using the Gifford-McMahorn cycle. Each refrigerator has what is termed a regenerator which is packed with what is termed regenerative material. A working medium (.sup.4 He gas) is repeatedly passed through the regenerator to obtain a low temperature. More specifically, the working medium is first compressed and then made to flow in one direction through the regenerator. As the medium flows through the regenerator, heat energy is transferred from the medium to the regenerative material. When the medium flows out of the regenerator, it is expanded and its temperature is lowered further. The working medium is then made to flow in the opposite direction, through the regenerator again. This time heat energy is transferred from the regenerative material to the medium. The medium is passed twice, back and forth, through the regenerator in one refrigeration cycle. This cycle is repeated, thereby obtaining a low temperature.
The thermal characteristics of the regenerative material (sometimes referred to as its "recuperativeness"), and most significantly its specific heat, are the determinant of the efficiency of the refrigerator. The greater the recuperativeness regenerative materials have, the higher the heat-efficiency of each refrigeration cycle.
The regenerative materials used in the conventional regenerators are sintered particles of lead or mesh of copper or bronze or phosphor bronze. These regenerative materials exhibit a very small specific heat at extremely low temperatures of 20.degree. K. or less. Hence, they cannot accumulate sufficient heat energy at extremely low temperatures in each refrigeration cycle of the gas-cycle refrigerator. Nor can they supply sufficient heat energy to the working medium. Consequently, a gas-cycle refrigerator which has a regenerator filled with such regenerative materials has a low cooling efficiency.
This problem can be solved by using regenerative materials which exhibit a large specific heat per unit volume (i.e., volume specific heat) at extremely low temperatures. Attention has been focused on some kinds of magnetic substances as such regenerative materials because their entropies greatly change at their magnetic phase transition temperature and show an anomalous specific heat (large specific heat). Hence, a magnetic substance that has an extremely low magnetic phase transition temperature can make an excellent regenerative material.
One such magnetic substance is the R-Rh intermetallic compound (where R is selected from the group consisting of: Sm, Gd, Tb, Dy, Ho, Er, Tm, and Yb). This material is disclosed in Japanese Patent Disclosure (Tokkai-sho) No. 51-52378. This group of intermetallic compounds has a maximal value of volume specific heat which is sufficiently great at 20.degree. K. or less.
One of the components of this intermetallic compound is rhodium (Rh). Rhodium is a very expensive material and thus is not suitable as a regenerative material used in a regenerator where the regenerator may weigh in an amount of hundreds of grams.
Another regenerative material R-Mz (where R is selected from the group consisting of: Se, Y, La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, and M is selected from the group consisting of: Ni, Co, Cu, Ag, Au, Mn, Fe, Al, Zr, Pd, B, Si, P, C, and z has a value in the range of: 0.001&lt;z&lt;9.0) has a large specific heat below 20.degree. K. and is relatively inexpensive. Such a material is disclosed in Japanese Patent Disclosure (Tokkai-hei) No. 1-310269 corresponding to U.S. Ser. No. 305,169 filed Feb. 2, 1989 (now abandoned), the parent of U.S. Ser. No. 536,083 filed Jun. 11, 1990 (now abandoned), the parent of U.S. Ser. No. 804,501 filed Dec. 10, 1991 (pending).
The regenerative material R-Mz, however, does not have sufficient specific heat at extremely low temperature (4.degree. K-5.degree. K).
For a Helium refrigerator especially, one of the most important factors governing the refrigeration efficiency is that the regenerative material have a high specific heat at the intended temperature of operation of the refrigerator.
Accordingly, one of the objects of the present invention is to provide a regenerative material which has a maximum specific heat at low temperature.
Another object of the present invention is to provide a low-temperature regenerator which is filled with the regenerative material described above.